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Magnetotelluric constraints on the role of fluids in convergent plate boundaries Open Access


Other title
Subduction zone
Continent-continent collision
Channel flow
Canadian Cordillera
Tibetan Plateau
Dehydration melting
Type of item
Degree grantor
University of Alberta
Author or creator
Rippe, Dennis
Supervisor and department
Unsworth, Martyn (Physics, Earth and Atmospheric Sciences)
Examining committee member and department
Currie, Claire (Physics)
Gu, Jeffrey (Physics)
Bailey, Richard (Physics, University of Toronto)
Waldron, John (Earth and Atmospheric Sciences)
Kravchinsky, Vadim (Physics)
Department of Physics
Date accepted
Graduation date
Doctor of Philosophy
Degree level
Convergent plate boundaries have played an important role in the growth and assembly of the modern continents. To obtain a better understanding of the tectonic processes in these boundaries, it is necessary to constrain the rheology of the crust and upper mantle in these regions. Magnetotelluric studies measure electrical resistivities and provide an excellent tool to determine the fluid content and thermal structure. These are key parameters in determining the rheology. Magnetotelluric studies were used to investigate two types of convergent plate boundaries: (i) the Cascadia subduction zone as an example for active subduction of an oceanic plate beneath a continent and (ii) the Indian-Eurasian collision as an example for a modern continent-continent collision. Geodynamic models requiring vigorous convection of a low viscosity mantle have been successful in explaining high mantle temperatures in the backarc of the Cascadia subduction zone. Geochemical calculations indicate that high temperatures and water content can significantly reduce mantle viscosity. Long-period magnetotelluric data of the Cascadia subduction zone are used to determine the electrical resistivity structure and to constrain the backarc mantle rheology, specifically in terms of the amount of fluids. Non-uniqueness in the interpretation is reduced by using constraints from seismic tomography and geochemical calculations. At shallow mantle depths, water contents of 500-1000 ppm and melt fractions of 0.5-2.5% are required, which can reduce mantle viscosity and allow for vigorous mantle convection. In the India-Eurasia continental collision, models invoking flow in a weak lower crustal layer have been successful in explaining a number of geological and geophysical observations associated with the evolution of the Tibetan plateau. Geophysical observations indicate that the weak lower crustal layer may be the result of partial melting and/or aqueous fluids. Laboratory studies predict an order reduction in strength for melt fractions of 5-10%. By relating these laboratory studies to magnetotelluric observations it is possible to estimate the flow parameters associated with the channel flow model. Magnetotelluric studies require conductances of up to 20,000 S beneath the Tibetan Plateau, suggesting flow velocities of 0.02-4.5 cm/a. These flow velocities support the idea that channel flow could occur beneath parts of Tibet.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Rippe, D. & Unsworth, M. (2010). Quantifying crustal flow in Tibet with magnetotelluric data. Physics of the Earth and Planetary Interiors, 179(3-4), 107-121, doi: 10.1016/j.pepi.2010.01.009.

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File title: Magnetotelluric constraints on the role of fluids in convergent plate boundaries
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